<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>单列布局</title>
  <style>
    *{
      margin:0;
      padding:0;
    }
    header{
      height:50px;
      background:black;
    }
    main{
      max-width:960px;
      height:500px;
      margin:0 auto;
      background:red;
    }
    footer{
       height:50px;
       background:yellow;
    }
  </style>
</head>
<body>
  <header></header>
  <main></main>
  <footer></footer>
</body>
</html>
